A fiber optic force sensing catheter

Field of the Invention

0001The disclosed invention relates generally to force sensing devices capable of resolving the magnitude and direction of a force vector. More specifically, the invention relates to a force sensing tip to aid in the positioning of catheters used in humans or animals, or for serving as feedback elements in robotic surgical systems.

Background

0002For many years, exploration and treatment of various organs or vessels has been possible using catheter-based diagnostic and treatment systems. Such catheters are introduced through a vessel leading to the cavity of the organ to be explored or treated or alternatively may be introduced directly through an incision made in the wall of the organ. In this manner, the patient avoids the trauma and extended recuperation times typically associated with open surgical procedures.

0003To provide effective diagnosis or therapy, it is frequently necessary to first map the zone to be treated with great precision. Such mapping may be performed, for example, when it is desired to selectively ablate current pathways within a heart to treat atrial fibrillation. Often, the mapping procedure is complicated by difficulties in locating the zone(s) to be treated due to periodic movement of the heart throughout the cardiac cycle.

0004Previously-known systems for mapping the interior of a vessel or organ are described, for example, in <patcit id="pcit0001" dnum="US6546271B"><text>U.S. Patent Nos. 6,546,271</text></patcit> and <patcit id="pcit0002" dnum="US6226542B"><text>6,226,542</text></patcit>. The catheters described in those patents employ electromagnetic, electrical, magnetic or acoustic sensors to map the position of a distal end of the catheter in space and then construct a three-dimensional visualization of the vessel or organ interior.

0005One drawback of such previously known mapping systems is that they rely on manual feedback of the catheter and/or impedance measurements to determine when the catheter is properly positioned in the vessel or organ. Those systems do not measure contact forces with the vessel or organ wall or detect contact forces applied by the catheter against the organ or vessel wall that may modify the true wall location. Instead, previously known mapping methods are time-consuming, dependent upon the skill of the clinician, and cannot compensate for artifacts created by excessive contact forces.

0006It therefore would be desirable to provide apparatus and methods for detecting and monitoring contact forces between a mapping catheter and the wall of the organ or vessel to permit faster and more accurate mapping. It also would be desirable to provide apparatus and methods that permit the process to be automated.

0007Once the topography of the vessel or organ is mapped, either the same or a different catheter may be employed to effect treatment. Depending upon the specific treatment to be applied to the vessel or organ, the catheter may comprise any of a number of end effectors, such as but not limited to RF ablation electrodes, rotary or scissor action cutting heads, laser ablation system, injection or sewing needles, fluid conveyance systems, forceps, manipulators, mapping electrodes, endoscopic vision systems and therapeutic delivery systems such as genetic impregnation devices. Exemplary systems are described, for example, in <patcit id="pcit0003" dnum="US6120520A"><text>U.S. Patent Nos. 6,120,520</text></patcit>, <patcit id="pcit0004" dnum="US6102926A"><text>6,102,926</text></patcit>, <patcit id="pcit0005" dnum="US5575787A"><text>5,575,787</text></patcit>, <patcit id="pcit0006" dnum="US5409000A"><text>5,409,000</text></patcit> and<patcit id="pcit0007" dnum="US5423807A"><text> 5,423,807</text></patcit>.

0008The effectiveness of such end effectors often depends on having the end effector in contact with the tissue of the wall of the organ or vessel. Many previously-known treatment systems include expandable baskets or hooks that stabilize the distal extremity of the catheter in contact with the tissue. Such arrangements, however, may be inherently imprecise due to the motion of the organ or vessel. Moreover, the previously-known systems do not provide the ability to sense the load applied to the distal extremity of the catheter by movement of the tissue wall.

0009For example, in the case of a cardiac ablation system, at one extreme the creation of a gap between the end effector of the treatment system and the tissue wall may render the treatment ineffective, and inadequately ablate the tissue zone. At the other extreme, if the end effector of the catheter contacts the tissue wall with excessive force, it may inadvertently puncture the tissue, resulting in cardiac tamponade.

0010In view of the foregoing, it would be desirable to provide a catheter-based diagnostic or treatment system that permits sensing of the load applied to the distal extremity of the catheter, including periodic loads arising from movement of the organ or tissue. It further would be desirable to have a load sensing system coupled to control operation of the end effector, so that the end effector is operated, either manually or automatically, only when the contact force is detected to fall within a predetermined range.

0011<patcit id="pcit0008" dnum="US6695808B"><text>U.S. Patent No. 6,695,808</text></patcit> proposes several solutions to measure the force vector arising from contact with the tissue surface, including mechanical, capacitive, inductive and resistive pressure sensing devices. One drawback of such devices, however, is that they are relatively complex and must be sealed to prevent blood or other liquids from disturbing the measurements. In addition, such load sensing devices may result in an increase in the insertion profile of the distal extremity of the catheter. Still further, sensors of the types described in that patent may be subject to electromagnetic interference.

0012One previously-known solution for dealing with potential electromagnetic interference in the medical environment is to use light-based systems rather than electrical measurement systems. One such light-based system is described in <patcit id="pcit0009" dnum="US6470205B"><text>U.S. Patent No. 6,470,205</text></patcit> to Bosselman which describes a robotic system for performing surgery comprising a series of rigid links coupled by articulated joints. A plurality of Bragg gratings are disposed at the articulated joints so that the bend angle of each joint may be determined optically, for example, by measuring the change in the wavelength of light reflected by the Bragg gratings using an interferometer.

0013International Publication No. <patcit id="pcit0010" dnum="WO0133165A"><text>WO 01/33165 to Bucholtz </text></patcit>describes an alternative spatial orientation system wherein wavelength changes measured in a triad of optical fiber strain sensors are used to compute the spatial orientation of a catheter or other medical instrument. Although Bucholtz discloses that the strain sensors may be encased within a deformable sheath, as is also described in Bosselman, calculation of the bend angles is not described as requiring characterization of the material properties of the deformable sheath.

0014Accordingly, it would be desirable to provide diagnostic and treatment apparatus, such as a catheter or guide wire, that permits sensing of loads applied to a distal extremity of the apparatus, but which do not substantially increase the insertion profile of the apparatus. It is further desirable to provide diagnostic and treatment apparatus, such as a catheter and guide wire, that permits computation of forces applied to a distal extremity of the apparatus, and which are substantially immune to electromagnetic interference.

0015Recent advances in catheter technology have included the use of fiber optic force sensors to detect the reactive force at the distal extremity of an end effector when placed in contact with the interior wall of a vessel or organ. For example, an article by <nplcit id="ncit0001" npl-type="b"><text>J. Peirs et al., entitled "Design of an Optical Force Sensor for Force Feedback during Minimally Invasive Robotic Surgery," published by Katholieke Universiteit Leuven, Belgium</text></nplcit>, describes a tri-axial force sensor for use generating force feedback systems in a robotic surgery system. The apparatus includes a plurality of optical fibers that direct light onto a mirrored surface disposed adjacent to a distal tip of the device. The intensity of the light reflected from the mirrored surface is measured and may be correlated to the force required to impose a predetermined amount of flexure to the distal tip. The article describes a flexible and compact structure that may be used to produce variations in light intensity responsive to contact forces that deform the structure.

0016International Publication No. <patcit id="pcit0011" dnum="WO2007015139A"><text>WO 2007/015139 to Leo, et al.</text></patcit> (Leo), discloses a device and method for resolving a force vector (magnitude and direction) applied to the distal end of a catheter. Leo discloses the use of fiber optic strain elements in a catheter that maintains essentially the same profile as with catheters that do not sense touching forces and is substantially immune to electromagnetic interference. United States Patent Application Publication No. <patcit id="pcit0012" dnum="US20070060847A"><text>2007/0060847 to Leo et al.</text></patcit> discloses a force sensing catheter system that utilizes the deformation of fiber Bragg grating strain sensors to infer the force incident upon the tip of the catheter. United States Patent Application Publication No. <patcit id="pcit0013" dnum="US20080009750A"><text>2008/0009750 to Aeby et al.</text></patcit> discloses a tri-axial force sensor having a deformable structure that isolates the deflections caused by forces imposed on the distal end of the catheter and wherein fiber optics both irradiate and receive reflected light from the deformable structure, with intensities of the received reflected light varying according to the imposed force. The two-part form of claim 1 is based on this document. United States Patent Application Publication No. <patcit id="pcit0014" dnum="US20080294144A"><text>2008/0294144 to Leo et al.</text></patcit> discloses a fiber optic touch sensing catheter that utilizes an interferometric principle to detect structural deformations of a strain sensing assembly to infer forces. The above mentioned Publications to Leo et al. and Aeby et al. are assigned to the assignee of the present application. for

0017Existing fiber optic strain sensing catheters are typically limited to resolving forces to within approximately ±1-gm of force. In addition, the tri-axial force sensors tend to involve complex machining and fabrication to achieve the desired isolation effect. A fiber optic touch sensing catheter having greater sensitivity (higher resolution) and that is relatively easy to fabricate would be welcome.

Summary of the Invention

0018The invention is disclosed in claims 1 and 4 with preferred embodiments disclosed in the dependent claims.

0019Various embodiments of the invention include a structural member that improves the sensitivity of the force resolution over existing fiber optic strain sensing catheters by up to an order of magnitude. Some embodiments are further characterized as having a reduced profile over existing devices.

0020Structurally, the flexures of the various embodiments of the present invention include a cross-section having a stiffness (area moment of inertia) about a first axis that is on the order of 20 times less than the stiffness of a second axis that is orthogonal to the first axis. The relatively small stiffness about the first axis causes the flexure to bend preferentially due to moments about the first axis while bending about the second, orthogonal axis is minimal, thereby transferring the torsional forces about the second orthogonal axis onto other portions of the structural member. In this way, the flexures enable isolation of moments and moment forces about the first axis.

0021In various embodiments, a structural member defines a longitudinal axis and includes a plurality of segments that are adjacent each other in a serial arrangement along the longitudinal axis. Adjacent segments may define a gap therebetween, each gap being bridged by a flexure. A plurality of fiber optics operatively coupled with the structural member. The flexure may define a cross-section on a plane orthogonal to the longitudinal axis, the cross-section defining an area centroid, and a first inertial axis and a second inertial axis may be defined as passing through the area centroid. The second inertial axis may be normal to the first inertial axis and intersect the longitudinal axis of the structural member. The area moment of inertia about the second inertial axis may be at least ten times greater than the area moment of inertia about the first inertial axis.

0022Certain embodiments of the invention include a flexible elongated body having a proximal end and a distal extremity. A fiber optic force sensing assembly may be disposed within the flexible elongated body proximate the distal extremity, the fiber optic force sensing assembly comprising a structural member having an outer surface and defining a longitudinal axis. The structural member may also include a plurality of segments that are adjacent each other in a serial arrangement along the longitudinal axis, the segments being bridged by flexures located between adjacent of the segments. Each of the flexures may define a portion of the outer surface of the structural member. The plurality of segments may further define a plurality of gaps, each of the plurality of gaps being located between adjacent of the plurality of segments.

0023A plurality of fiber optics may be disposed on the structural member, each of the plurality of fiber optics having a distal end disposed adjacent one of the plurality of gaps and oriented for emission of light onto and for collection of light reflected from the segment adjacent the one of the plurality of gaps. The distance between the distal end of the fiber optic and the segment adjacent the one of the plurality of gaps has a dimension that varies in response to a degree of deformation of the structural member when a contact force is imposed on the structural member.

0024The distal end of said fiber optic and said segment adjacent said one of said plurality of gaps may define an interferometric resonator for inference of the distance of deflection.

Brief Description of the Drawings

0025<ul id="ul0001" list-style="none"><li><figref idref="f0001">FIG. 1</figref> is a block diagram of a strain sensing system in an embodiment of the invention;</li><li><figref idref="f0001">FIG. 1A</figref> is a schematic depiction of an interferometric fiber optic sensor in for use in an embodiment of the invention;</li><li><figref idref="f0001">FIG. 1B</figref> is a schematic depiction of a fiber Bragg grating optical sensor in for use in an embodiment of the invention;</li><li><figref idref="f0002">FIG. 2</figref> is a partial cutaway view of a distal portion of a catheter assembly having a fiber optic force sensing assembly in an embodiment of the invention;</li><li><figref idref="f0003">FIG. 3</figref> is an enlarged perspective view of a fiber optic force sensing assembly that utilizes an intensity or an interferometric measurement in an embodiment of the invention;</li><li><figref idref="f0004">FIG. 4</figref> is an elevation view of the fiber optic force sensing assembly of <figref idref="f0003">FIG. 3</figref>;</li><li><figref idref="f0005">FIGS. 5 through 8</figref> are sectional views of the fiber optic force sensing assembly of <figref idref="f0004">FIG. 4</figref>;</li><li><figref idref="f0006">FIG. 9</figref> is an enlarged sectional view of a flexure defining a circular segment in an embodiment of the invention;</li><li><figref idref="f0006">FIG. 10</figref> is a partial enlarged view of the fiber optic force sensing assembly of <figref idref="f0003">FIG. 3</figref>;</li><li><figref idref="f0007">FIGS 11A and 11B</figref> depict the deflection of the fiber optic force sensing assembly of <figref idref="f0003">FIG. 3</figref> under an axial load and a lateral load, respectively;</li><li><figref idref="f0008">FIG. 12</figref> is an enlarged perspective view of a fiber optic force sensing assembly that utilizes a Fabry-Perot strain sensor in an embodiment of the invention;</li><li><figref idref="f0008">FIG. 12A</figref> is a sectional view of a Fabry-Perot strain sensor of <figref idref="f0008">FIG. 12</figref>;</li><li><figref idref="f0009">FIG. 13</figref> is an elevation view of the fiber optic force sensing assembly of <figref idref="f0008">FIG. 12</figref>;</li><li><figref idref="f0009">FIGS. 14 through 16</figref> are sectional views of the fiber optic force sensing assembly of <figref idref="f0009">FIG. 13</figref>;</li><li><figref idref="f0010">FIG. 17</figref> is an enlarged perspective partial cutaway view of a second fiber optic force sensing assembly that utilizes a fiber Bragg grating strain sensor in an embodiment of the invention;</li><li><figref idref="f0011">FIG. 18</figref> is an elevation view of the fiber optic force sensing assembly of <figref idref="f0010">FIG. 17</figref>; and</li><li><figref idref="f0011">FIGS. 19 through 21</figref> are sectional views of the fiber optic force sensing assembly of <figref idref="f0011">FIG.18</figref>.</li></ul>

Detailed Description

0026Referring to <figref idref="f0001">FIG. 1</figref>, an embodiment of a strain sensing system 70 is depicted in accordance with the invention. The strain sensing system 70 may comprise an electromagnetic source 72, a coupler 74, a receiver 76, an operator console 77 operatively coupled with a microprocessor 78 and a storage device 79. The electromagnetic source 72 outputs a transmitted radiation 80 of electromagnetic radiation that is substantially steady state in nature, such as a laser or a broadband light source. A transmission line 82 such as a fiber optic cable carries the transmitted radiation 80 to the coupler 74, which directs the transmitted radiation 80 through a transmitting / receiving line 84 and through a fiber optic element 83 (<figref idref="f0001">FIG. 1A</figref>) contained within a flexible, elongate catheter assembly 87 to a fiber optic sensing element 90. The fiber optic element 83 of the catheter assembly 87 and transmitting / receiving line 84 may be coupled through a connector 86 as depicted in <figref idref="f0001">FIG. 1</figref>.

0027The catheter assembly 87 may have a width and a length suitable for insertion into a bodily vessel or organ. In one embodiment, the catheter assembly 87 comprises a proximal portion 87a, a middle portion 87b and a distal portion 87c. The distal portion 87c may include an end effector 88 which may house the fiber optic sensing element 90. The catheter assembly may be of a hollow construction (i.e. having a lumen) or of a nonhollow construction (i.e. no lumen), depending on the application.

0028Referring to <figref idref="f0001">FIG. 1A</figref>, an interferometric fiber optic strain sensor 90a is depicted as the fiber optic strain sensing element 90 in an embodiment of the invention. In this embodiment, the transmitted radiation 80 enters an interferometric gap 85 within the interferometric fiber optic strain sensor 90a. A portion of the radiation that enters the interferometric gap 85 is returned to the fiber optic cable of the catheter assembly 87 as a modulated waveform 89a. The various components of the interferometric fiber optic strain sensor 90a may comprise a structure that is integral the fiber optic element 83 (e.g., <figref idref="f0008">FIG. 12A</figref>). Alternatively, the fiber optic element 83 may cooperate with the structure to which it is mounted to form the interferometric gap 85 (e.g., <figref idref="f0006">FIG. 10</figref>).

0029Referring to <figref idref="f0001">FIG. 1B</figref>, a fiber Bragg grating strain sensor 90b is depicted as the fiber optic strain sensing element 90 in an embodiment of the invention. In this embodiment, the transmitted radiation 80 enters a fiber Bragg grating 91, the gratings of which are typically integral with the fiber optic element 83 and reflect only a portion 89b of the transmitted radiation 80 about a central wavelength λ. The central wavelength λ at which the portion 89b is reflected is a function of the spacing between the gratings of the fiber Bragg grating. Therefore, the central wavelength λ is indicative of the strain on the fiber Bragg grating strain sensor 90b relative to some reference state.

0030The reflected radiation 89, be it the modulated waveform 89a or the reflected portion 89b, is transmitted back through the transmitting / receiving line 84 to the receiver 76. The strain sensing system 70 may interrogate the fiber optic strain sensing element 90 at an exemplary and non-limiting rate of 10-Hz. The receiver 76 is selected to correspond with the type of strain sensing element 90 utilized. That is, the receiver in the depicted embodiments is selected to either detect the frequency of the modulated waveform 89a for use with the interferometric fiber optic strain sensor 90a, or to resolve the central wavelength of the reflected portion 89b for use with fiber Bragg grating strain sensor 90b. The receiver 76 manipulates and/or converts the incoming reflected radiation 89 into digital signals for processing by the microprocessor 78.

0031Referring to <figref idref="f0002">FIG. 2</figref>, an example of the end effector 88 comprising an ablation head 88a and including a fiber optic force sensing assembly 92 is depicted in an embodiment of the invention. The fiber optic force sensing assembly 92 may be configured to structural deformations in a structural member 102 caused by a force F imposed on a distal extremity 94 of the catheter, e.g., when distal extremity 94 contacts the wall of a bodily vessel or organ.

0032It is understood that one or more end effectors 88 of different kinds, e.g., mapping electrodes or ablation electrodes, such as are known in the art for diagnosis or treatment of a vessel or organ may be utilized with the invention. For example, the catheter assembly 87 may be configured as an electrophysiology catheter for performing cardiac mapping and ablation. In other embodiments, the catheter assembly 87 may be configured to deliver drugs or bioactive agents to a vessel or organ wall or to perform minimally invasive procedures such as transmyocardial revascularization or cryo-ablation.

0033Referring to <figref idref="f0003 f0004 f0005 f0006">FIGS. 3 through 10</figref>, the fiber optic force sensing assembly 92 including the structural member 102 and a plurality of fiber optics 104 is depicted in an embodiment of the invention. In this embodiment, the structural member 102 defines a longitudinal axis 110 and includes an outer surface 112. The structural member 102 is divided into a plurality of segments 116, identified in <figref idref="f0003 f0004 f0005 f0006">FIGS. 3 through 10</figref> as a base segment 118, a proximal segment 120, a middle segment 122 and a distal segment 124. The segments 116 may be adjacent each other in a serial arrangement along the longitudinal axis 110.

0034The segments 116 may be bridged by a plurality of flexure portions 128, identified individually as flexure portions 128a, 128b and 128c, thus defining a plurality of neutral axes 130, identified individually as neutral axes 130a, 130b and 130c. Each neutral axis 130 constitutes the location within the respective flexure portion 128 that the stress is zero when subject to pure bending in any direction.

0035In one embodiment, adjacent members of the segments 116 may define a plurality of gaps 136, each having a separation dimension. For clarity, the gaps 136 are identified as 136a through 136c. The separation dimensions of the gaps 136a, 136b and 136c may be of the same approximate magnitude (as depicted) or of different magnitudes (not depicted). It is further noted that while the separation dimensions of the gaps 136 are depicted as being uniform, the separation dimension may vary in the lateral direction across a given gap 136a, 136b, 136c. Each gap 136a, 136b and 136c, whether of a uniform or a non-uniform separation dimension, may define a corresponding central plane 138a, 138b and 138c located equidistant between adjacent ones of the segments 116.

0036The structural member 102 may include a plurality of grooves 142 (identified in <figref idref="f0003 f0004 f0005 f0006">FIGS. 3 through 10</figref> as grooves 142a, 142b and 142c) that are formed on the outer surface 112. The grooves 142 may be spaced rotationally equidistant (i.e. spaced 120° apart) about the longitudinal axis 110 and may be oriented in a substantially axial direction along the structural member 102. Each of the grooves may terminate at a respective one of the gaps 136. For example, the groove 142a may extend along the base segment 118, the proximal segment 120 and the middle segment 122, terminating at the gap 136a. Likewise, groove 142b may extend along the base segment 118 and the proximal segment 120, terminating at the gap 136b. And groove 142c may extend along the base segment 118, terminating at the gap 136c.

0037The fiber optics 104 (identified in <figref idref="f0003 f0004 f0005 f0006">FIGS. 3 through 10</figref> as fiber optics 104a, 104b and 104c) define a plurality of light propagation axes 148 and distal ends 150 (identified in <figref idref="f0003 f0004 f0005 f0006">FIGS. 3 through 10</figref> as distal ends 148a through 148c and 150a through 150c, respectively). The fiber optics 104 may be disposed in the grooves 142 (identified in <figref idref="f0003 f0004 f0005 f0006">FIGS. 3 through 10</figref> as 142a, 142b and 142c) such that the distal ends 150 terminate at the gaps 136. For example, the fiber optic 104a may extend along the groove 142a, terminating proximate or within the gap 136a. Likewise, fiber optics 104b and 104c may extend along the grooves 142b and 142c, respectively, terminating proximate or within the gaps 136b and 136c, respectively.

0038By the above described arrangement, each of the light propagation axes 148 of the fiber optics 104 are subtended by a respective one of the segments 116. For example, as depicted in <figref idref="f0006">FIG. 9</figref>, the light propagation axis 148a is subtended by a surface 154a of the distal segment 124, which defines the boundary of the gap 136a opposite the distal end 150a. Accordingly, the surfaces 154 of the segments 116 that are proximate the subtended light propagation axes 148 may be made highly reflective.

0039The gaps 136 may be formed so that they extend laterally through a major portion of the structural member 102. Also, the gaps 136 may be oriented to extend substantially normal to the longitudinal axis 110 (as depicted) or at an acute angle with respect to the longitudinal axis. In the depicted embodiment, the structural member comprises a hollow cylindrical tube 156 with the gaps 136 comprising slots 158 that are formed from one side of the hollow cylindrical tube 156 and are transverse to the longitudinal axis 110, extending through the longitudinal axis 110 and across the inner diameter 160 of the hollow cylindrical tube 156 to a depth 162 (<figref idref="f0005">FIG. 5</figref>).

0040By this process, the flexure portions 128 remain, defining a circular segment. The depth 162 of the slots 158 traverse the inner diameter 160 of the hollow cylindrical tube 156 can be varied to establish a desired flexibility of the flexure. That is, the greater the depth 162 the more flexible the flexure portion 128. The slots 156 may be formed by the various ways available to the artisan, such as but not limited to sawing, laser cutting or electro-discharge machining (EDM).

0041The slots 158 may be formed so that the flexure portions 128 define non-coincident neutral axes 130. That is, neutral axis 130a is located at a circumferential position about the longitudinal axis 110 that substantially differs from the circumferential position of the neutral axes 130b and 130c. The neutral axes 130 may be, but need not be, diametrically opposed to the location of the distal end 150 of the fiber optic 104 that terminates in the same gap 136 as is bridged by the respective flexure portion 128. For example, flexure portion 128a may be diametrically opposed to distal end 150a, and so on.

0042A cross-section 164 of the flexure portions 128 is depicted in <figref idref="f0006">FIG. 9</figref>. The cross-section is characterized by an area centroid C that corresponds with the neutral axis 130, and as having inertial axes x-x and y-y that are orthogonal, and where the inertial axis x-x identifies the axis about which the area moment of inertia is minimum. The circular segment geometry provides substantially greater stiffness about inertial axis y-y than about inertial axis x-x. Consider a circular segment having an angle of π/2 radians (90°). The area moment of inertia about the inertial axis y-y is about twenty times greater than the area moment of inertia about the inertial axis x-x. Accordingly, forces that cause a moment about inertial axis y-y will typically cause very little bending relative to the same moment being applied about inertial axis x-x. Therefore, moments about inertial axis y-y will tend to be transferred as a torsional force between adjacent sections, whereas moments about inertial axis x-x will tend to cause a deflection.

0043A deflection beam length 163 is defined as the distance between the neutral axis 130 and the center of the distal end 150 of the corresponding fiber optic 104, the distance being normal to the inertial axis x-x. By locating the distal end 150 and the neutral axis 130 of a given gap 136 in diametric opposition, the deflection beam length 163 is maximized, and so is the attendant change in the dimension of the gap proximate the distal end 150.

0044In one embodiment, each gap 136 enables an interferometric gap 166 to be defined between the distal end 150 of the respective fiber optic 104 and the high reflective surface 154. An "interferometric gap" as used herein is a gap having the attributes an interferometric resonator, such as found in a Michelson interferometer or a Fabry-Perot resonator. Likewise, a "gap interferometer" as used herein is an interferometer that utilizes an interferometric gap to produce an interference pattern.

0045The interferometric gap 166 may be characterized as having an operative length 167, defined as the distance between the distal end 150 and the high reflective surface 154 and which may differ from the dimension of the respective gap 136. The operative length 167 establishes the characteristics of the interference pattern reflected back from the interferometric gap 166. The distal ends 150 may be faced with a semi-reflecting surface or coating 168 that re-reflects a portion of the light reflected from the high reflective surface 154 while substantially transmitting the remaining portion of the reflected light therethrough for detection by the strain sensing system 70.

0046In another embodiment, light is transmitted and reflected back across the respective gaps 136, with the reflected light being collected by the distal end 150 of the respective fiber optic 104. The intensity of the reflected light collected by a given fiber optic 104 may vary with the distance between the distal end 150 and the high reflective surface 154. Embodiments that utilize the variation of reflected light intensity may utilize distal ends 150 that are exposed rather than face with the semi-reflecting surface or coating 168, thereby increasing the amount of light that can be detected.

0047In one embodiment, the fiber optics 104 are bonded to the structural member 102 with an adhesive or bonding material 170. Alternatively or in addition, the fiber optics 104 may be press fit or otherwise fastened to the structural member 102. For configurations that employ an intensity or interferometric measurement, the fiber optic 104 may be bonded to the segment 116 adjacent the respective gap 136 to be interrogated. For example, fiber optic 104b may be mounted within the portion of groove 142b that is formed on the middle segment 122. The remainder of the fiber optic 104b may be left to slide freely within the remainder of the groove 142b. By this arrangement, the fiber optic 104b will not form a structural bridge between adjacent segments, which would inhibit the flexibility of the fiber optic force sensing assembly 92.

0048Referring to <figref idref="f0007">FIGS. 11A and 11B</figref>, operation of the fiber optic force sensing assembly 92 in response to an axial force FA and a lateral force FL, respectively, is depicted in an embodiment of the invention. The axial force FA causes the segments 116 to bend about the inertial axes x-x of the various flexure portions 128 in substantially a pure bending action, thus causing the dimension of the gaps 136 proximate the distal ends 150 of the fiber optics 104 to decrease (<figref idref="f0007">FIG. 11 A)</figref>. This, in turn, causes the operative lengths 167 of the interferometric gaps 166 to decrease, thereby causing a change in the frequencies of the interferometric patterns sustained across the interferometric gaps 166.

0049The lateral force FL will generally cause a more complex deformation of the structural member 102. In the depiction of <figref idref="f0006">FIG. 10B</figref>, the lateral force FL is applied substantially parallel to the inertial axis y-y of flexure portion 128a. This causes flexure portion 128a to translate a moment between distal segment 124 and middle segment 122 while causing a negligible change in the dimension of gap 136a. The translated moment causes flexure portions 128b and 128c to bend about their respective inertial axes x-x, which in turn causes the gap 136b to close proximate distal end 150b of fiber optic 104b and the gap 136c to open proximate the distal end 150c of fiber optic 104c. It is noted that in the depiction of <figref idref="f0007">FIG. 11B</figref>, neither flexure portion 128b or 128c are in pure bending because lateral force FL does not act normal to the respective inertial axes x-x. Hence, the degree of bending about the inertial axes x-x will generally be proportional to the component of the lateral force FL that acts normal thereto.

0050It is understood that <figref idref="f0007">FIGS. 11A and 11B</figref> show a purely axial and a purely lateral force, respectively, but that a combined force vector in three-dimensional space having an axial and a lateral component will combine the general effects depicted by superposition. Accordingly, a force vector in three-dimensional space can be resolved by calibrating the response of the fiber optic force sensing assembly under these pure loads and superimposing the various responses to infer the axial and lateral components.

0051The characteristics of the modulated waveform 89a (<figref idref="f0001">FIG. 1A</figref>) are determined in part by the dimension of the interferometric gap 85. The fiber optic force sensing assembly 92 is configured so that the interferometric gap 85 will vary when the structural member 102 experiences an axial strain. A change in the axial strain will cause a proportional change in the dimension of the interferometric gap 85, thereby altering the characteristic of the modulated waveform 89a transmitted to the receiver 76.

0052The preceding embodiments can provide a mechanical amplification of the change in the interferometric gap 85 relative to the strain experienced by the flexure portions 128. The deflection of the segments 116 at a position normal to the inertial axis x-x of a respective one of the flexure portions 128 is proportional to the deflection beam length 163 between the neutral axis 130 and the respective location of the distal end 150 of the respective fiber optic 104. Accordingly, change in the dimension of the gap 136 will be greatest at a location that is diametrically opposed to the neutral axis 130. Accordingly, for embodiments where the distal ends 150 of the fiber optics 104 in diametric opposition to the neutral axes 130 (as depicted herein), the fiber optics 104 are in a position of greatest sensitivity.

0053The structural member 102 may be fabricated from other forms besides a hollow cylindrical tube, including but not limited to tubes or rods that define a square, rectangular or cross-shaped cross-section. The structural member 102 may comprise a metallic material, such as titanium or platinum/iridium, or a non-metallic material such as a polymer or ceramic. The gaps 136 and flexure portions 128 may be sized so that the change in the operative lengths 167 due to application of forces FA and FL is of substantially higher sensitivity than the change caused thermal expansion or contraction of the structural member 102 under operation.

0054Furthermore, the material for structural member 102 may be selected to mitigate against the effects of thermal expansion. For example, the structural member 102 may be constructed of a material having a low coefficient of thermal expansion, such as fused quartz, aluminum oxides such as alumina (Al₂O₃), liquid crystal polymer, or from metal/ceramic composites such as Invar designed to for a low coefficient of thermal expansion relative to metals.

0055The adhesive or bonding material 170 may comprise a glue or epoxy. The bonding material 170 may be selected to closely match the coefficient of thermal expansion (CTE) of the structural member 102 and/or fiber optics 104, or to provide a CTE that is between the CTEs of the structural member 102 and fiber optics 104 to provide a transition therebetween. The bonding material 170 may also be chosen for flexibility so that the thermal growth of the adhesive film does not impose a substantial strain on the fiber optics 104. Use of a very thin film of bonding material 170 may, in some instances, mitigate the effects of differential thermal expansion.

0056Where the structural member 102 comprises a polymer material, the fiber optics 104 may be bonded directly to the polymer using a bonding technique that involves the use of a solvent designed to cause the polymer to melt or flow while not affecting the material of the fiber optics 104. The solvent may be applied to an area or zone of the structural member 102 where the fiber optics 104 are to be mounted, and the fiber optics 104 placed thereon. Alternatively, the fiber optics 104 may be temporarily held in place on the zone or area of the structural member 102 and the solvent applied to both. The flowing of the material causes a bond between the structural member 102 and the outer surface of the fiber optics 104. The solvent may be removed by a process such as washing or evaporation to arrest the melting process.

0057The reflective surfaces 154 may be fabricated by polishing a metallic structural member 102, or by depositing a reflective material on either of a metallic or a non-metallic structural member 102. Representative and non-limiting dimensions for the structural member are approximately 1- to 10- mm in length, approximately 0.3- to 3- mm in diameter, and gap dimensions of approximately 15- to 100- micorometers.

0058Referring to <figref idref="f0008">FIGS. 12, 12A</figref> and <figref idref="f0009">FIGS. 13 through 16</figref>, a fiber optic force sensing assembly 192 including a structural member 196 is depicted in an embodiment of the invention. The structural member 196 includes many of the same aspects as the structural member 102 of <figref idref="f0003 f0004 f0005 f0006">FIGS. 3 through 9</figref>, which are labeled in the respective figures with the same numerical references. The fiber optic force sensing assembly 192 includes fiber optics 202 (identified in <figref idref="f0008 f0009">FIGS. 12 through 16</figref> as fiber optics 202a, 202b and 202c) each operatively coupled to a respective one of a plurality of Fabry-Perot strain sensors 198 (identified as 198a, 198b and 198c). Fabry-Perot strain sensors of the general configuration presented in <figref idref="f0008">FIG. 12 A</figref> are commercially available from FISO Technologies of Quebec, QC, Canada.

0059The operation of the Fabry-Perot strain sensors 198 is depicted in <figref idref="f0008">FIG. 12A</figref>. The fiber optic 202 is split into a transmitting element 204a and a reflecting element 204b, each being anchored at opposing ends of a hollow tube 206. The transmitting and reflecting elements 204a and 204b are positioned to define an interferometric gap 205 therebetween having an operative length 207. The free end of the transmitting element 204a may be faced with a semi-reflecting surface 200a, and the free end of the reflecting element 204b may be faced with a reflecting surface 200b.

0060The fiber optics 202 may be positioned along the grooves 142 so that the respective Fabry-Perot strain sensor 198 is bridged between by the respective fiber optic 202 and across segments 116 that are adjacent each other. For example, fiber optic 202a may be positioned within groove 142a so that the Fabry-Perot strain sensor 198a bridges the gap 136a between the middle segment 122 and the distal segment 124. Likewise, fiber optics 202b and 202c may be positioned so that Fabry-Perot sensors 198b and 198c to bridge the gaps 136b and 136c, respectively.

0061The fiber optic 202 may be operatively coupled to both the adjacent segments 116 that the fiber optic 202 bridges. The fiber optics 202 may be attached using the adhesive or bonding material 170 to the segments 116, as best seen in <figref idref="f0009">FIGS. 14 through 16</figref>. In this embodiment, the bonding material 170 is applied only to the segments 116 that are adjacent the gap 136 bridged by the fiber optic 202. For example, fiber optic 202a is attached only to the middle and distal segments 122 and 124 (<figref idref="f0009">FIGS. 14 and 15</figref>), but not to the proximal segment 120 (<figref idref="f0009">FIG. 16</figref>) or the base segment 118. The grooves 142 may also extend the full length of the structural member 196, as depicted in <figref idref="f0008">FIG. 12</figref>, as there is no need for the structural member 196 to reflect light back into the fiber optics 202.

0062Referring to <figref idref="f0010 f0011">FIGS. 17 through 21</figref>, a fiber optic force sensing assembly 208 is depicted in an embodiment of the invention. The fiber optic force sensing assembly 208 utilizes fiber optics 202 having a fiber Bragg grating strain sensor 214. In this embodiment, the fiber optics 202 are disposed on an interior surface 210 of a structural member 212. Accordingly, the structural member 212 need not include grooves that run axially on the exterior surface. Otherwise, the fiber optics 202 are aligned along the length of the structural member 212 and attached to the interior surface 210 so that the fiber Bragg grating strain sensors 214 fixedly bridge adjacent segments 116. Again, the fiber optics 202 may be affixed to the interior surface 212 using the bonding material 170.

0063In operation, when a force is applied at or near the distal extremity 94 of either of the structural members 196 or 212, the structural member 196, 212 will have a tendency to flex, for example, as depicted in <figref idref="f0007">FIGS. 11A and 11B</figref>. However, the fiber optics 202 being fixedly attached to adjacent segments 116 causes a restorative or reactive force that limits deflection between the segments 116 of the structural member 196. The reactive forces, which may be a compressive force or a tension force, causes a strain across the strain sensors of the fiber optics 202 (Fabry-Perot strain sensors 198 of the fiber optic force sensing assembly 192 or fiber Bragg grating strain sensors 214 of the fiber optic force sensing assembly 208).

0064The fiber optic force sensing assembly 192 may be configured so that the axial force and strain exerted on the fiber optics 202 is at a desired level. For example, the diameter of the fiber optics 202 will have an effect on the strain imposed on the fiber Bragg grating strain sensor 214, with smaller diameter fiber optics providing a greater strain per unit of axial force. Also, the geometry of the flexure portions 128 will affect the magnitude of the force that is transferred to the fiber optics 202. That is, a flexure portion 128 having a greater stiffness (i.e. a greater area moment of inertia about the inertial axis x-x) will transfer less force to the respective fiber optic 202. Moreover, the magnitude of the reactive force will vary with the normal distance between the fiber optic 202 and the inertial axis x-x, with the reactive force generally increasing as the normal distance decreases. Hence, the reactive force for a single fiber optic located at the deflection beam length 163 (<figref idref="f0005">FIG. 7</figref>) will be less than for a single fiber optic located on the structural element 102 that defines a normal distance between the optic 202 and the inertial axis x-x that is shorter than the deflection beam length 163.

0065By tweaking these various parameters, the strain sensed by a fiber optic strain sensor may be tailored to provide a desired sensitivity. In terms of the reactive forces, it is contemplated that the ratio of the flexure axial force to the fiber optic axial force can range from 0.2 to 5.

0066It is noted that while the depicted embodiments present a single fiber optic diametrically opposed to the neutral axis of a respective flexure (e.g., fiber optic 202a in diametric opposition to neutral axis 130a), the invention is not so limited. The fiber optics of the various embodiments may be located on the structural body at a location other than diametric opposition with respect to the neutral axis of a given flexure. Moreover, the number of fiber optics utilized for detecting the forces exerted may be greater than one. For example, a configuration utilizing a fiber optic strain sensor (e.g., fiber Bragg grating strain sensors 214) may include a pair of fiber optic strain sensors, each located at a circumferential location on the structural member that defines restorative moment arms about the inertial axis x-x that are of equal magnitude. It is further noted that the Fabry-Perot detection schemes and the fiber Bragg grating detection schemes may each be present on the same fiber optic force sensing assembly.

0067For the fiber optic force sensing assembly 208, the strain causes the interferometric gap of the Fabry-Perot strain sensor 198 to change and the frequency of the returned modulated waveform to shift in frequency. The frequency change can be calibrated to correspond to the reactive force using known techniques. For the fiber optic force sensing assembly 208, the strain causes a shift in the central wavelength of the light that is reflected by the respective fiber Bragg grating strain sensor 214, which can be calibrated to correspond to the reactive force using known techniques.

0068The invention may be practiced in other embodiments not disclosed herein, such as endoscopic or additional intravascular applications. For example, various aspects of the disclosed embodiments may be utilized in a diagnostic catheter for optimizing or otherwise improving the placement of excitation electrodes for baroreflex activation. Other aspects of the disclosed embodiments may find application in endoscopic applications, such as orthoscopic surgery or entry through open orifices such as the throat, nose or anus without departing from the the invention.

0069References to relative terms such as upper and lower, front and back, left and right, or the like, are intended for convenience of description and are not contemplated to limit the invention, or its components, to any specific orientation. All dimensions depicted in the figures may vary with a potential design and the intended use of a specific embodiment of this invention without departing from the scope thereof.

0070Each of the additional figures and methods disclosed herein may be used separately, or in conjunction with other features and methods, to provide improved devices, systems and methods for making and using the same. Therefore, combinations of features and methods disclosed herein may not be necessary to practice the invention in its broadest sense and are instead disclosed merely to particularly describe representative embodiments of the invention.